A rotary energy converter of this type, or hydrostatic rotary fluid pump/motor, has two members which are rotated relative to one another to increase or decrease the volume of a space for intaking or discharging fluid. Thus, the converter can be used either as a pump or as a motor. A support element that supports one of the two members is reciprocated at right angles to an axis of rotation to adjust the distance between the centers of the two members for changing the displacement of the converter. This structure is customarily called a variable-displacement type structure.
Recently, hydraulically driven systems of the power recovery type have been required to maintain a constant pressure regardless of whether discharging or intaking fluid. In this respect, the energy converter of the variable displacement type is equipped with a pressure-compensating mechanism, and the displacement can be varied such that when the load pressure reaches a certain preset value, the discharge of the pump is adjusted according to the discharge pressure. Such energy converter performs the control over the pressure which is now described in more detail by referring to FIG. 4. When the pump operates at a certain flow rate a, if the discharge pressure increases to a preset pressure, then the pressure-compensating valve opens, gradually reducing the discharge rate as indicated by b. Thus, the displacement varies automatically to a non-discharging condition. Consequently, the present situation is that an energy converter of this kind cannot be switched between pump and motor at will, nor can it recover energy efficiently.